This invention is concerned with evaporative light scattering detector (ELSD), especially concerned with a sheath-flow device for ELSD.
ELSD is a universal and mass sensitive detector. It is not susceptible to molecular structure and spectroscopic properties of a compound. ELSD is suitable for detection of any compounds that have a lower volatility than the mobile phase, therefore, it can be used for volatile and semi-volatile compounds, especially suitable and powerful for detection and analysis of natural product, traditional Chinese medicine (TCM), lipid, surfactant and hydrocarbon, etc. Since ELSD has little effect on the peak broadening of micro-flow chromatography, it is suitable for any chromatographic techniques based on capillary column.
Micro-fluid evaporative light-scattering detector (pELSD) is a miniaturized device that matches nano-liter to micro-liter/min flow of the mobile phase in a capillary chromatographic system. Its principle is as follows: 1) the effluent from the capillary column is nebulized into an aerosol, then, 2) the solvent in the aerosol is evaporated and the solute is left as tiny particles, then, 3) the particles gets into the detection cell where they scatter the photos from the light beam, and then, 4) a photomultiplier detects the photos and convert the optical signal into an electronic signal.
Because of the extremely low flow rate of the capillary column, the quantity of the solute compounds after nebulization and evaporation into the detection cell is very low and easy to spread, and resulting in low detection sensitivity and low stability. In addition, the low flow rate of the small amount of solute aerosol is susceptible to form eddy and turbulence flow, resulting in un-symmetric chromatographic peak.
Sheath flow is an auxiliary flow that can be either sheath liquid or sheath gas flow. The essence of the sheath-flow is to focus the effluent, which is similar to the sheath flow used in cytometer to avoid blood cell to bypass the counter.
There is a sheath flow device in conventional ELSD but it is not feasible to be used in pELSD in which the flow is only a few microliters/min. The conventional sheath flow device is not able to prevent heat transform to the detection cell from the evaporative tube.